Hydrogel implant with superficial pores

ABSTRACT

Implantable biomaterials, particularly hydrogel substrates with porous surfaces, and methods for enhancing the compatibility of biomaterials with living tissue, and for causing physical attachment between biomaterials and living tissues are provided. Also provided are implants suitable for load-bearing surfaces in hard tissue repair, replacement, or augmentation, and to methods of their use. One embodiment of the invention relates to an implantable spinal disc prosthesis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/053,409, filed Feb. 7, 2005, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/542,389, filed Feb. 6, 2004,both of which are incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to spinal disc replacement devices, particularlydevices which mimic native spinal discs, for implantation which iscompatible with living tissue. The invention also relates to implantssuitable for load-bearing surfaces in the repair of tissue, replacementor augmentation, and methods of using such. One embodiment of theinvention relates to an implantable spinal disc prosthesis.

2. Background Art

Materials used in the construction of implantable medical devices mustbe nontoxic, nonantigenic, and noninflammatory. Hydrogels are apreferred type of polymeric material for implantable devices. Because oftheir high water content, analogous to living tissue, they are superiorin biocompatibility to non-hydrous polymeric materials.

U.S. Pat. No. 5,981,826, issued to Ku et al., describes the preparationof polyvinyl alcohol hydrogels (PVA-H) by physically crosslinking anaqueous solution of polyvinyl alcohol (PVA) to produce a gel. Thecrosslinking is accomplished by subjecting the aqueous PVA solution tomultiple cycles of freezing and thawing. One limitation of the prior artis that the hydrogels produced are relatively nonporous and the poresize and degree of porosity, that is the density of the pores within thehydrogel, cannot vary independently of the mechanical properties orstiffness of the hydrogel.

Methods for producing certain porous hydrogels also exist in the art.U.S. Pat. No. 6,268,405, issued to Yao et al., describes methods forcreating porous PVA-Hs by including immiscible materials in thepolymerization process. After the hydrogel is polymerized, the includedimmiscible materials are washed out of the hydrogel by an appropriatesolvent, yielding pores which are broadly distributed throughout thehydrogel. Controlling the size and density of the pores is accomplishedby varying the molecular weight of the immiscible materials. Adisadvantage of Yao et al. is that the range of attainable pore sizes islimited. Moreover, the invention of Yao et al. is limited in that it canonly produce hydrogels whose pores extend throughout the hydrogel. Thepores in Yao et al. are intended to create vascularization of thehydrogel in soft or non-load bearing tissue. A further disadvantage ofYao et al. is that the pore sizes are broadly distributed about theaverage pore size.

Artificial discs intended for the replacement of a damagedintravertebral disc have been described. These are typically articulateddevices comprising two rigid metal plates adhered to opposite ends of anelastomeric core. In use, the artificial disc is placed in theintervertebral space and the metal plates are secured to the surfaces ofadjacent vertebrae. Various embodiments of artificial discs of this typeare described in U.S. Pat. Nos. 5,674,296 and 6,156,067, issued to Bryanet al., U.S. Pat. No. 5,824,094, issued to Serhan et al., U.S. Pat. No.6,402,785, issued to Zdeblick et al. More recent embodiments, e.g. U.S.Pat. No. 6,419,704, issued to Ferree and U.S. Pat. No. 6,482,234, issuedto Weber et al., include descriptions of elastomeric cores that may beformed from materials with different elasticities to better mimic thenative structure of spinal discs.

Artificial discs have also been described wherein the disc is comprisedof a flexible urethane silicone blend core and two identical rigidsurfaces on either side of the core. U.S. Pat. No. 6,607,558 to Kurasdescribes such a disc where nail like projections extend from thesurface to fixate the disc between the vertebrae. Such a disc alsopossesses a different material for the end plates as for the elasticcore.

The disadvantage of the artificial disc devices of the prior art arenumerous. These prior art devices require the mechanical attachment ofrigid artificial materials, such as titanium, directly to the bone withscrews, staples, nails, cement, or other mechanical means. These rigidmaterials are only minimally compatible with natural, living bone andseparation of the implant from the bone is often observed over long-termimplantation. In addition, materials used in artificial discs of theprior art have physical and mechanical properties distinctly differentfrom those of natural spinal discs and thus inadequately duplicate thedesired properties of native spinal discs.

Vertebral fusion is still the most commonly performed procedure to treatdebilitating pain associated with degenerative spinal disc disease ordisc trauma, despite the fact that the procedure has many drawbacks.Vertebral fusion increases stress and strain on the discs adjacent tothe fusion site, and it is now widely accepted that fusion isresponsible for the accelerated degeneration of adjacent levels. Currentmulticomponent spinal disc prosthesis designs, elastomeric cores withmetal plates on both the upper and lower surfaces, are susceptible toproblems with interfacial bonding and wear. These designs have shownspontaneous device detachment due to retraction of bone tissue from themetal surface.

Bone ingrowth and attachment in the art has often required the use ofbone promoting growth factors. For example, U.S. Pat. No. 5,108,436,issued to Chu et al., describes using a porous implant for use in loadbearing bone replacement which is used in combination with an osteogenicfactor such as TGF-β.

Biomedical devices which are implanted in or around bone often failbecause of fibrinogen encapsulation of the implant instead of cellularattachment to the implant itself. This encapsulation is a defensivereaction attempting to minimize contact between the body and the implantand is considered a sign of implant incompatibility.

Moreover, the art of bone ingrowth onto implantable surface contains amultitude of examples relating to porous directed ingrowth where boneessentially grows into and around channels of the implant. For example,U.S. Pat. No. 4,911,720, issued to Collier et al., discusses theingrowth of bone into interconnecting pores which essentially locks boneinto place. This method is disadvantageous in that bone does notactually attach itself to the material, instead bone attaches to otherbone around the implant. In the unfortunate event that an implant mustbe removed, this type of Collier ingrowth results in large amounts ofdisruption to the surrounding bone tissue.

SUMMARY OF THE INVENTION

The present invention describes a hydrogel for implantation into a loadbearing space within the body. The hydrogel has a textured surface on itwhich is comprised of superficial surface pores. Stated differently, thepores on the surface of the hydrogel substrate do not extend throughoutthe hydrogel but instead remain within a region near the surface. Thepores on this hydrogel substrate can have an average diameter of between1 and 100 micrometers. Preferably the average diameter of surface poreson the hydrogel substrate is between 5 and 50 micrometers, andpreferably between 10 and 30 micrometers. The superficial pores of thishydrogel substrate can vary in size by less than 50%, preferably lessthan 30%, and preferably less than 10%. The hydrogel substrate of thepresent invention can be made up of polyvinyl alcohol and can have awater content of at least 5% w/w of the overall hydrogel. The hydrogelsubstrate of the present invention could be used in any load bearingimplantable device application including, but not limited to, a spinaldisc replacement. The present invention when used as a spinal discreplacement can possess the surface pores in the proper configurationand be additionally comprised of multiple regions of varyingelasticities. It is also possible that the regions of varyingelasticities of the spinal disc replacement be comprised of multiplehydrogels as opposed to one hydrogel of varying elasticities.

The present invention also includes a method for making a hydrogelsubstrate with a textured surface for use in a load bearingbiocompatible device. The hydrogel in liquid form is exposed to solidobjects or to a mold which when polymerized or hardened results in ahydrogel with a textured surface. The solid objects used to impart thesuperficial pores may be made of polystyrene beads. Also, the solidobjects used to impart the superficial pores may be grit, sand, silicon,silica, and ultra-fine particulate matter. The solid objects used tocreate the superficial pores can have a diameter of between 1 and 100micrometers, preferably between 5 and 50 micrometers, and preferablybetween 10 and 30 micrometers. The solid objects used to create thesuperficial pores of this invention can be removed, for example, by useof an organic solvent or other washing means. This hydrogel substratecan be comprised of poly-vinyl alcohol possessing a water content of atleast 5% w/w of the overall hydrogel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an elevation view of spinal disc replacement made inaccordance with one embodiment of the present invention.

FIG. 2 is a schematic of a surface generated in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is drawn to an implantable hydrogel substratecomprising a hydrogel surface having a plurality of superficial poresthereon. The pores on this hydrogel substrate can have an averagediameter of between 1 and 100 micrometers. Preferably the averagediameter of surface pores on the this hydrogel substrate is between 5and 50 micrometers, and preferably between 10 and 30 micrometers. Thesuperficial pores of this hydrogel substrate can vary in size by lessthan 50%, preferably less than 30%, and preferably by less than 10%. Thehydrogel substrate of the present invention can be made up of polyvinylalcohol and can have a water content of at least 5% w/w of the overallhydrogel.

One embodiment of the present invention is an artificial disc forimplantation into the spine comprising the hydrogel substrate describedabove. This artificial disc is a hydrogel possessing a plurality ofregions having variable elasticity. Specifically, the disc is comprisedof a surface region having a higher modulus of elasticity than aninterior region. This disc can be prepared using multiple hydrogels withthe elastic properties of the one disc with varying regions ofelasticity.

The present invention also includes a method for making a hydrogelsubstrate comprising contacting solid objects with an aqueous hydrogel,allowing the hydrogel to polymerize and crosslink while the solidobjects are at least partially immersed in the hydrogel, and removingthose solid objects from the polymerized and crosslinked hydrogel toform superficial pores thereon. The solid objects used to impart thesuperficial pores may be polystyrene beads. Alternatively, the solidobjects used to impart the superficial pores may be grit, sand, silicon,silica, and ultra-fine particulate matter. The solid objects used tocreate the superficial pores and therefore the pores themselves can havea diameter of between 1 and 100 micrometers, preferably between 5 and 50micrometers, and preferably between 10 and 30 micrometers.

The solid objects used to create the superficial pores of this inventioncan be removed for example by use of an organic solvent or other washingmeans. This hydrogel substrate can be comprised of poly-vinyl alcoholpossessing a water content of at least 5% w/w of the overall hydrogel.

Accordingly, the present invention is directed to an implantablehydrogel substrate product, a method of making that product, and amethod of using that product which substantially improves upon thelimitations existing in the art. To achieve these and other advantagesin accordance with the purpose of the invention, as embodied and broadlydescribed herein, the invention includes a load bearing biocompatiblehydrogel for medical implantation that promotes bone attachment. Thehydrogel substrate consists of a hydrogel surface component which hasbeen optimized for implantation. This is accomplished through pores onthe surface having a controlled range in distribution of size. Thesurface pores are superficial and do not extend throughout the hydrogel.

Hydrogels are materials whose state is between that of a solid and of aliquid. Gels consist of polymeric, i.e. long chain, molecules linkedtogether to form a three-dimensional network and are embedded in aliquid medium. In the case of hydrogels, the liquid medium compriseswater. The polymer backbone of hydrogels is formed by hydrophilicmonomer units and may be neutral or ionic. Examples of neutral andhydrophilic monomer units are ethylene oxide, vinyl alcohol,(meth)acrylamide, N-alkylated (meth)acrylamides,N-methylol(meth)acrylamide, N-vinylamides, N-vinylformamide,N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-methylformamide,hydroxyalkyl (meth)acrylates such as hydroxyethylmethacrylate,vinylpyrrolidone, (meth)acrylic esters of polyethylene glycol monoallylethers, allyl ethers, of polyethylene glycols, and sugar units such asglucose or galactose. Examples of cationic hydrophilic monomer units areethyleneimine (in the protonated form), diallyldimethylammonium chlorideand trimethylammonium propylmethacrylamide chloride. Examples of anionicmonomer units are (meth)acrylic acid, crotonic acid, maleic acid,fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid,vinylsulfonic acid, vinylphosphonic acid,2-methacryloyloxyethanesulfonic acid, 4-vinylbenzenesulfonic acid,allylsulfonic acid, vinyltoluenesulfonic acid and vinylbenzenephosphonicacid.

From the example listing above, a hydrogel for use in the presentinvention may be selected based upon its biocompatibility and stabilityat various hydration states. For the purposes of the present invention,a suitable hydrogel will have a moisture content of at least 5% w/w ofthe overall hydrogel, preferably at least 10%, 15%, 20%, 25%, 30%, 35%,40%, 50%, 60%, 70%, or 80% w/w of the overall hydrogel.

Initial events following implantation of a biomaterial in an orthotopicsurgical site include rapid adsorption of serum constituents onto theimplant surface. The first cells that are likely to come into contactwith the surface are polymorphonuclear cells, platelets, monocytes, andmacrophages. These cells release bioactive factors that promotemesenchyrnal cell migration to the wound site. In addition to thesenatural factors associated with wound healing, surgeons frequently usebone graft and bone graft substitutes to improve bone formation. Suchmaterials include osteoinductive agents such as demineralized bonematrix and bone morphogenetic protein. If appropriate signals arepresent mesenchymal cells with an osteoprogenitor phenotype willcontinue to differentiate into osteoblasts; of these a subset willbecome osteocytes. Ultimately, the newly formed bone will be remodeledvia osteoclastic resorption. The present invention also provides thatwell-known grafting agents may be incorporated into the hydrogelcomposition, which includes, but is not limited to growth factors,angiogenic agents, antibiotics, and the like.

Chemically modified or polar surfaces are generally known to be able toproduce more reactive protein adsorption to the implant surface thanunmodified or non-polar surfaces. The increased reactivity of theproteins adsorbed onto the polar surface is thought to promote cellularadhesion to that surface. Therefore, the invention provides that thehydrogel composition can possess chemically modified or polar surfaces.

In general, many materials are well-tolerated in bone, but the successof long-term or chronic implantation often depends on the intimacy ofthe interface between the material surface and the bone.Microarchitecture of the surface is an important determinant of cellresponse. It has been observed that osteoblast phenotypic expression issurface-dependent. As described herein, specific surface characteristicsenhance osteoblast differentiation while permitting proliferation,leading to optimal cell response to the implantation.

The mechanical properties of the material must be appropriate for theapplication. When the mechanical properties of the material are similarto the mechanical properties of the tissue adjacent to the implant,tissue tolerance of the artificial material is enhanced. Polymeric andelastomeric biomaterials can be fabricated with a wide range ofmechanical properties, making them suitable for many applications asimplantable devices. Because of their high water content, similar tothat of living tissue, hydrogels are superior in biocompatibility tonon-hydrous polymeric materials. Poly-vinyl alcohol (PVA) is an exampleof a polymer that can be used to form hydrogels, and has been studiedextensively for its potential in biomedical applications. Poly-vinylalcohol hydrogels (PVA-Hs) are biologically well tolerated andcompatible with living cartilage tissue.

PVA-Hs can be produced from solution via repeated freezing and thawingcycles that increase the order of the microcrystalline regions, changingthe dissolution properties, mesh size, and diffusion properties of thepolymer. Also, PVA-Hs can be produced from solution via a slow andsustained transition through the freezing point of the solution. Themechanical properties of PVA-Hs can be varied over a wide range, andstable PVA gels can easily be produced to have an elastic modulusranging from a few MPa, such as articular cartilage, to about 50 MPa,such as the stiffest portion of the annulus of spinal discs.

Increasing the porosity of a hydrogel substrate produces decreasedmechanical strength. When porous hydrogels are used to provide therequisite surface of the present invention, it is advantageous that theporosity not extend throughout the hydrogel, but be limited to arelatively shallow depth below the surface. The thickness of the porousportion of the hydrogel is preferably less than 1 millimeter, less than500 micrometers, and most preferable less than or equal to 200micrometers.

The hydrogel substrates of the present invention can be used forimplantation into any part of the body. One embodiment of this inventionis an artificial intervertebral disc, comprising one or more hydrogelsshaped substantially similarly to a natural intervertebral disc. Theupper and lower surfaces of the hydrogel, or assembly of hydrogels, areconstructed to have a rouges or textured surface with a definedporosity. That porosity depends primarily upon the size of a solidobject used to create the surface texture. The surface texture iscreated by the distribution of pores that do not continue throughout thehydrogel, or, in other words, the pores are superficial. The size of thepores can be from 2 to 100 micrometers, preferably from 5 to 50micrometers, and preferably, from 10 to 30 micrometers.

The porosity of the hydrogel surface embodied in this invention may berealized in a variety of ways. Molds may be constructed with patterningon the appropriate surfaces of the cavities in the mold. Alternatively,the porosity may be produced by abrasion of a smooth hydrogel surfaceafter molding. Abrading the surface with grit will result in a surfacetextured such as desired in this invention. Techniques for applying andusing abrasives are well known to those of skill in the art.

One technique for producing the surface roughness of artificial discs ofthis invention involves providing solid objects having the size andshape of the required surface rugosity and then using these solidobjects as a template for the construction of a mold. Alternatively,these solid objects may be embedded in the hydrogels of the presentinvention during the molding process and removed afterwards. Removal ofthe solid objects leaves behind on the surface of the hydrogel pores,cavities, and other invaginations required for the texture of thesurface to be obtained.

One example of a material that can be used as solid objects to impartthe surface texture on the hydrogels of the present invention ispolystyrene. Polystyrene beads, commonly called latex beads, arecommercially available in sizes ranging from 0.02 to 1000 micrometers,and can have a very narrow size distribution. Such a narrow sizedistribution is advantageous for applications requiring uniform surfaceroughness. For example, when using polystyrene beads with an averagediameter of 20.3 μm an acceptable range for the distribution of beadsize would be .+−.0.6 μm, preferably .+−.0.5 μm, and preferably .+−.0.4μm. Polystyrene beads may be obtained having surface functional groupsuseful for covalent chemical attachment of various chemical entities tothe surface of the beads. Polystyrene beads may also be obtained eithercrosslinked or uncrosslinked. The latter type of beads are insoluble inwater, but freely soluble in many organic solvents. Thus, one method forremoval of the beads after the molding process is the dissolution of thebeads in appropriate organic solvents.

The pores on the textured surface in this embodiment enable the surfaceto resemble native bone which has undergone osteoclastic resorption.When surface textured hydrogels are used to provide the requisitesurface porosity, it is advantageous for the pores not to extendthroughout the hydrogel, but instead be limited to a relatively shallowdepth below the textured surface. The thickness of the porous portion ofthe hydrogel surface is preferably less than 1 millimeter, preferablyless than or equal to 500 micrometers, and preferably less than or equalto about 200 micrometers.

The hydrogels of the present invention may contain bioactive factors tofurther stimulate cell growth or differentiation. These factors, forinstance attachment peptides, such as RGD containing peptides, andgrowth factors such as bone morphogenic proteins, insulin-like growthfactor, platelet derived growth factor, fibroblast growth factor,cartilage-derived growth factor, transforming growth factor-beta, andparathyroid hormone related peptide, as well as other regulatorychemicals such as statins, prostaglandins, and mineral ions well-knownin the art. These factors may be included in the hydrogels of thisinvention singly or in combination, and they may be included with orwithout binding proteins.

The hydrogels of the present invention may also contain bone orcartilage forming cells (osteoblasts or chondrocytes) or precursor cellsto bone and cartilage forming cells such as mesenchymal stem cells orosteoprogenitor cells. These precursor cells have the capacity todifferentiate into bone and/or cartilage forming cells. Cells may beincluded in the hydrogels of the present invention alone or incombination with bioactive factors to further stimulate cell growth ordifferentiation.

Natural intervertebral discs have a tough outer fibrocartilaginous ringcalled the annulus fibrosus and a soft, inner, highly elastic structurecalled the nucleus pulpous. The artificial discs of the presentinvention may contain an inner core constructed to mimic the physicaland mechanical properties of the natural nucleus pulpous, surrounded byan annular region constructed to mimic the physical and mechanicalproperties of the natural annulus fibrosus.

In one embodiment, these regions comprise hydrogels whose water content,degree of polymerization, and degree of crosslinking are adjusted toproduce the requisite physical and mechanical properties. The hydrogelcomprising the inner core has a higher water content and/or a lowerdegree of polymerization and/or a lower degree of crosslinking toproduce a relatively soft and elastic hydrogel. The hydrogel comprisingthe outer annular region has a lower water content and/or a higherdegree of polymerization and/or crosslinking to produce a relativelyhard outer hydrogel which mechanically is tough and stiff. The hydrogelscomprising the upper and lower surfaces may substantially resemble thehydrogel comprising the annular region in terms of physical andmechanical properties, water content, and degrees of crosslinking andpolymerization. The additional requirement, however, for the surfaces tobe porous may allow or require a different combination of physical andmechanical properties in these hydrogels compared to the hydrogelcomprising the outer annular region.

FIG. 1 shows a spinal disc replacement envisioned by the presentinvention. The spinal disc has an upper portion 1 and a lower portion 2.It is the hydrogel substrate surfaces of the upper portion 1 and lowerportion 2 which possess the porous texture of the present invention. Theupper portion 1 and lower portion 2 can be less elastic and more rigidthan the inner region 4 which seeks to mimic the nucleus pulpous.Likewise, the spinal disc may have an intermediate region of elasticity3 which further aids in the function of the spinal disc. Theintermediate region of elasticity 3 may or may not differ from theelasticity of either the inner region 4 or the upper portion 1 or lowerportion 2.

FIG. 2 shows the upper portion 1 of the spinal disc of FIG. 1 possessingsuperficial surface pores 10 of the present invention. The sizing ofthese pores as described herein promotes differentiation of cells intodesired tissues, such as bone or bone-like cells, and induces theattachment of those cells to the surface.

In one embodiment, the superficial pores can be created with solidpolystyrene beads of 20.7 μm to 19.9 μm of average diameter suspended inthe hydrogel solution, or coated on the appropriate surfaces of the moldprior to crosslinking of the hydrogel. After crosslinking, thepolystyrene beads are removed by dissolving them in a solvent such asdimethyl formamide or its equivalent that does not dissolve thehydrogel. Such treatment produces porosity and surface roughness withdimensions approximately equal to the diameter of the polystyrene beads.The invaginations left behind after removal of the polystyrene beadscontribute to the controlled surface texture desired in the presentinvention. The polystyrene beads of this embodiment may also be adheredto a surface and used as a part of a mold, or as a template in theconstruction of a mold.

In another embodiment of this invention, the solid object used in thecasting of a rough hydrogel surface is grit. Grit can be any solidobject that is small with a narrow size distribution. Examples of gritinclude sand, silica particles, silicone particles, metal shot, etc.Those skilled in the art would recognize the need to match the grit usedto the hydrogel along with the method of removing the grit leaving therough or porous surface.

A silica grit of appropriate size can be used to impart the proper levelof porosity on the surface of a hydrogel. As was seen with thepolystyrene, the sand is placed in the bottom of a mold and the aqueoushydrogel is poured into the mold. The hydrogel is allowed to crosslinkand polymerize and is then removed from the mold. The hydrogel is thenwashed to remove the grit leaving behind the invaginations and poreswhich make up the textured surface of the hydrogel implant. Grit mayalso be adhered to a surface and used as a part of a mold, or as atemplate in the construction of a mold.

In yet another embodiment of the present invention, the mold used in theformation of the hydrogel substrate can be any type of material aroundwhich the hydrogel forms. For example, the mold can be a series ofbeads, grit, filter screens, mesh, wires, glass tubing, and theequivalents of these materials or items. Once the hydrogel has beenallowed to form around the mold element, the mold is removed from thehydrogel manually, chemically, or any other means which will allow thehydrogel to remain intact once the mold has been removed.

In yet another embodiment of the present invention, the hydrogelsubstrate can be a load bearing patch which can be used in the repair ofpartially or predominately damaged tissue. For example, the hydrogelsubstrate bearing the textured surface of the present invention can berelatively thin and small in diameter. That hydrogel substrate can thenbe placed where deteriorated, either acutely or chronically, cartilagewas removed.

In yet another embodiment of the present invention, the hydrogelsubstrate can be assembled outside the body in a malleable form. Themalleable form of the hydrogel substrate can then be placed in theintended area, be it a spinal disc replacement, knee cartilagereplacement, shoulder bursa repair, or other use one skilled in the artwould foresee. Once in the proper position, the malleable hydrogelsubstrate could be hardened or polymerized via photopolymerization.Radiation curing or photopolymerization (photo-induced free radicalpolymerization) has become an important and useful technique forapplying and curing coatings, inks and adhesives. Radiation-curablecompositions typically comprise as essential components one or moreradiation-curable monomers and a photoinitiator. The compositions areapplied as a coating to various articles and surfaces and the monomersare polymerized to form a film by exposing the coating of theradiation-curable composition to radiation, typically ultraviolet (UV)or electron-beam radiation.

EXAMPLES Example 1 Attachment of Polystyrene Objects to a Surface

To a suspension of carboxyl-modified polystyrene beads (20.3 μm.+−.0.43μm diameter, Bangs Laboratories) in 20 nM MEW, pH 4.4 is added a 10-foldexcess of water-soluble carbodiimide, 1-ethyl-3-(d-dimethylaminopropyl)carbodiimide hydrochloride. After 15 minutes at room temperature, thebeads are washed twice by centrifugation and suspension in 20 mM HEPES,pH 7.5 and then resuspended in the same buffer. The resulting suspensionis added to the wells of a 24-well tissue culture plate made ofpolycarbonate, having an amino-modified surface (Nalge NuncInternational). After 60 minutes at room temperature, the unreactedbeads are decanted, and the wells are washed gently with deionizedwater. Microscopic analysis shows the bottom surface of the wells arecovered with a monolayer of polystyrene beads at a density ofapproximately 50%.

The surface with attached polystyrene objects of the previous examplemay be used as a template to fabricate a mold for providing the desiredporous surface of the hydrogels of the present invention. This may beaccomplished by making a metallic replica of a surface comprising aplurality of polystyrene objects using sputtering and/or metal platingtechniques and the like, all of which are well known to those of skillin the art. The metallic replica thus produced may be replicated againand reinforced with further metal or other components, again usingmethods well known to those skilled in the art. The result is a moldsuitable for producing the surface texture of the hydrogels of thepresent invention.

The polystyrene objects of the foregoing example may also be included inthe hydrogels of the present invention during the molding process.Subsequent removal of the polystyrene objects provides the controlledsurface porosity provided for the hydrogels of the present invention.This is illustrated in the following example.

Example 2 Construction of a PVA-H with Surface Topography forImplantation as a Spinal Disc Prosthesis

A two-part mold with inserts is used to produce the artificial discdepicted in FIG. 1. The upper and lower halves of the mold are firstseparated to mold the upper and lower surface regions of the disc. Thesurface of each corresponding upper and lower surface is created bysuspending in the aqueous hydrogel the objects for creating thesuperficial pores. That suspension is then poured into the well of amold at a depth no greater than the desired depth of the superficialpores. That layer of hydrogel is allowed to polymerize and crosslink.From that base layer creating the outermost layer of the correspondingsurface, the remainder of the bulk of the hydrogel substrate can bebuilt up by adding additional depth of aqueous hydrogel.

A 30% w/w poly(vinyl alcohol) solution is prepared by mixing poly(vinylalcohol) polymer (124,000-186,000 Av. MW, 99+% saponification, AldrichChemical Company) in sterile, deionized water. The polymer is dissolvedby heating the mixture in an autoclave at 120° C. for 30 minutes. To aportion of the viscous liquid solution is added 30% w/w of thepolystyrene objects from Example 1. This is mixed until a uniformsuspension is obtained. To each of the cavities in each half of the moldis added a sufficient amount of this suspension to coat the surfaces ofthe mold cavities to a thickness of 200 μm. Inserts are placed in eachcavity to spread the suspension across the surfaces of the cavities andmaintain the 200 μm thickness.

The two halves of the mold are then subjected to five cycles of freezingand thawing. In each cycle, the molds are placed in a freezer at about−20° C. for approximately 12 hours then removed from the freezer andplaced at room temperature for approximately 12 hours. The insertsdefining the first 200 μm thickness of the top portion of the uppersurface and the bottom portion of the lower surface are removed from themolds, and an additional amount of the 30% poly(vinyl alcohol) solution(without polystyrene objects) is added to the mold cavities. The amountadded is sufficient to increase the thickness of the top portion of theupper surface and the bottom portion of the lower surface to 1 mm.Inserts are placed in each cavity to spread the solution and maintainthe 1 mm thickness.

The two halves of the mold are subjected to five additional cycles offreezing and thawing as above. The inserts defining the top portion ofthe upper surface and the bottom portion of the lower surface areremoved from the molds, and annular inserts defining the shape of thecore region are placed in the cavities of the lower half of the mold. A20% w/w poly(vinyl alcohol) solution is prepared by mixing poly(vinylalcohol) polymer (89,000-98,000 Av. MW 99+% saponification, AldrichChemical Company) in sterile, deionized water and dissolving as above.The solution is filled into the annular inserts and the mold issubjected to five additional cycles of freezing and thawing. The annularinserts are removed, the two halves of the mold are assembled andclamped together, and the areas of the mold corresponding the annularregion are filled with 30% poly(vinyl alcohol) solution. The assembledmold is subjected to five more cycles of freezing and thawing. Themolded artificial discs are removed from the mold and immersed indimethyl formamide to dissolve the included polystyrene objects, therinsed three times with deionized water.

The artificial disc produced by the forgoing example has a soft andelastic inner core while the outer annular region and upper and lowersurfaces are relatively hard, tough, and stiff. The surface of theartificial disc is smooth, except on the top portion of the uppersurface and the bottom portion of the lower surface where the removal ofthe polystyrene objects produces a rough or rugose surface with aroughness of 20 μm. When artificial discs made according to thisprocedure are implanted into the intervertebral spaces of disectomizedrabbits, extensive bone growth onto the surface occurs within 3 weeks.

Although the invention has been described with reference to a particularpreferred embodiment with its constituent parts, features and the like,these are not intended to exhaust all possible arrangements, mechanicaland electrical equivalents, or features, and indeed many othermodifications and variations will be ascertainable to those of skill inthe art.

1. A biocompatible hydrogel configured for implantation into a joint, comprising: a top surface configured for attachment to a patient's native tissue; a bottom surface generally opposite of the top surface; a depth defined between the top surface and the bottom surface; superficial pores located within about 1 millimeter of the top surface of the hydrogel; wherein the superficial pores do not extend throughout an entire depth of the hydrogel; wherein said superficial pores enhance osteoblast differentiation and permit proliferation of osteoblasts; and wherein the hydrogel is load-bearing.
 2. The biocompatible hydrogel of claim 1, wherein the hydrogel is configured for implantation into at least one of a knee, shoulder or spine.
 3. The biocompatible hydrogel of claim 1, wherein the pores have an average cross-section of between about 1 to 100 micrometers.
 4. The biocompatible hydrogel of claim 1, wherein the pores have an average cross-section of between about 10 to 30 micrometers.
 5. The biocompatible hydrogel of claim 1, wherein the pores are within about 200 micrometers of the surface of the hydrogel.
 7. The biocompatible hydrogel of claim 1, wherein the hydrogel comprises polyvinyl alcohol.
 8. The biocompatible hydrogel of claim 1, wherein the hydrogel has a moisture content of at least 5% w/w of the overall hydrogel.
 9. The biocompatible hydrogel of claim 1, wherein the hydrogel has a moisture content of at least 30% w/w of the overall hydrogel.
 10. The biocompatible hydrogel of claim 1, wherein the hydrogel comprises a chemically-modified or polar surface to promote cellular adhesion.
 11. A load bearing hydrogel implant comprising: a first surface having a plurality of superficial pores thereon, said first surface being configured for attachment to a patient's native joint tissue; wherein said superficial pores are located at or near the first surface; wherein said superficial pores do not extend throughout said hydrogel; wherein the hydrogel is configured for implantation into a joint; wherein the superficial pores have an average cross-section of between about 1 and 100 micrometers; and wherein the pores are within about 1 millimeter of the hydrogel surface.
 12. The hydrogel implant of claim 11, wherein the hydrogel is configured for implantation into at least one of a knee, shoulder or spine.
 13. The hydrogel implant of claim 11, wherein the pores have an average cross-section of between about 1 to 100 micrometers.
 14. The hydrogel implant of claim 11, wherein the pores have an average cross-section of between about 10 to 30 micrometers.
 15. The hydrogel implant of claim 11, wherein the pores are within about 1 millimeter of the surface of the hydrogel.
 16. The hydrogel implant of claim 11, wherein the pores are within about 200 micrometers of the surface of the hydrogel.
 17. The hydrogel implant of claim 11, wherein the hydrogel comprises polyvinyl alcohol.
 18. The hydrogel implant of claim 11, wherein the hydrogel has a moisture content of at least 5% w/w of the overall hydrogel.
 19. The hydrogel implant of claim 11, wherein the hydrogel has a moisture content of at least 30% w/w of the overall hydrogel. 